The present invention relates to optical instruments and techniques for microscopic imaging using charged particles, in particular, to electron and ion microscopes.
Electron and ion microscopes are widely used in microscopic imaging. Image formation in the conventional electron (ion) microscope is a one-step process comprising the illumination of a specimen from a source of charged particles, the use of electron (ion) lenses for obtaining a magnified image, and the image recording by a detector. The resolving power of such a microscope is known to be limited by aberrations which inevitably accompany electron lenses.
Gabor's holographic electron microscope uses a two-step imaging process, consisting of recording a hologram of the specimen and reconstructing a magnified image, to compensate for such aberrations. The hologram recording requires spatial and temporal coherence of the interfering object and reference wave; therefore the condition that the spatial coherence width of the specimen illumination must be greater than the specimen width, is an essential feature of the holographic microscope. A modern modification of the holographic electron microscope employing "image-plane off-axis holography" (see, e.g., U.S. Pat. No. 4,935,635 (C1. 235-306) "Electron Holography Apparatus"), incorporated by reference, comprises a bright-field electron microscope having an electron source and condenser that provide an incident electron beam of spatial coherence radius greater than the transverse dimensions of the specimen which is located off-axis, electron lenses for obtaining a magnified image of the specimen, an electrostatic biprism for recombining the magnified image with the undisturbed part of the incident beam to form an image-plane hologram, an image detector for recording the hologram, and an image data processor used to analyze the hologram to reconstruct an image. However, the image resolution attainable with this microscope even after an aberration correction procedure, though improved compared to the resolution of conventional electron microscopes, cannot exceed the so-called information-resolution limit determined chiefly, among other factors, by the chromatic and spherical aberration of the objective lens. Another drawback of such a microscope is that for a three-dimensional specimen, the high-resolution image is not easily interpretable. This requires performing laborious image simulations on a computer.